Complex fluids are mixtures that have a coexistence between two phases: solid–liquid (

suspension Suspension or suspended may refer to: Science and engineering * Suspension (topology), in mathematics * Suspension (dynamical systems), in mathematics * Suspension of a ring, in mathematics * Suspension (chemistry), small solid particles suspende ...

s or solutions of macromolecules such as polymers), solid–gas (

granular Granularity (also called graininess), the condition of existing in granules or grains, refers to the extent to which a material or system is composed of distinguishable pieces. It can either refer to the extent to which a larger entity is subd ...

), liquid–gas (

foam Foams are materials formed by trapping pockets of gas in a liquid or solid. A bath sponge and the head on a glass of beer are examples of foams. In most foams, the volume of gas is large, with thin films of liquid or solid separating the ...

s) or liquid–liquid (

emulsion An emulsion is a mixture of two or more liquids that are normally immiscible (unmixable or unblendable) owing to liquid-liquid phase separation. Emulsions are part of a more general class of two-phase systems of matter called colloids. Altho ...

s). They exhibit unusual mechanical responses to applied stress or strain due to the geometrical constraints that the phase coexistence imposes. The mechanical response includes transitions between solid-like and fluid-like behavior as well as fluctuations. Their mechanical properties can be attributed to characteristics such as high disorder, caging, and clustering on multiple length scales.

Example

Shaving cream Shaving cream or shave cream is a category of cream cosmetics used for shaving preparation. The purpose of shaving cream is to soften the hair by providing lubrication. Different types of shaving creams include aerosol shaving cream (also kn ...

is an example of a complex fluid. Without stress, the foam appears to be a solid: it does not flow and can support (very) light loads. However, when adequate stress is applied, shaving cream flows easily like a fluid. On the level of individual bubbles, the flow is due to rearrangements of small collections of bubbles. On this scale, the flow is not smooth, but instead consists of fluctuations due to rearrangements of the bubbles and releases of stress. These fluctuations are similar to the fluctuations that are studied in

earthquake An earthquake (also known as a quake, tremor or temblor) is the shaking of the surface of the Earth resulting from a sudden release of energy in the Earth's lithosphere that creates seismic waves. Earthquakes can range in intensity, fr ...

Dynamics

The dynamics of the particles in complex fluids are an area of current research. Energy lost due to friction may be a nonlinear function of the velocity and normal forces. The topological inhibition to flow by the crowding of constituent particles is a key element in these systems. Under certain conditions, including high densities and low

temperature Temperature is a physical quantity that expresses quantitatively the perceptions of hotness and coldness. Temperature is measurement, measured with a thermometer. Thermometers are calibrated in various Conversion of units of temperature, temp ...

s, when externally driven to induce flow, complex fluids are characterized by irregular intervals of solid-like behavior followed by stress relaxations due to particle rearrangements. The dynamics of these systems are highly nonlinear in nature. The increase in stress by an infinitesimal amount or a small displacement of a single particle can result in the difference between an arrested state and fluid-like behavior. Although many materials found in nature can fit into the class of complex fluids, very little is well understood about them. Inconsistent and controversial conclusions concerning their material properties still persist. The careful study of these systems may lead to "new physics" and new states of matter. For example, it has been suggested that these systems can jam and a "jamming phase diagram" can be used to consider how these systems can jam and unjam. It is not known whether further research will demonstrate these findings, or whether such a theoretical framework will prove useful. As yet this large body of theoretical work has been poorly supported with experiments.

External links

Stephan Herminghaus' Dynamics of Complex Fluids DepartmentDavid Weitz's Soft Condensed Matter Physics LaboratoryHoward Stone's Complex Fluids GroupBob Behringer's complex fluids pageHernán Alejandro Makse's complex fluids pageComplex Fluids/Nonlinear Dynamics LaboratoryFrancois Graner's complex fluids pageCarnegie Mellon University Center for Complex Fluids Engineering

Paulo Arratia's Complex Fluids Laboratory at PennComplex Fluids & Computational Polymer Physics at ETH ZurichUbaldo M. Córdova-Figueroa's Low Reynolds Fluid Mechanics Group at UPRMZhengdong Cheng's Soft Condensed Matter Group
Fluid dynamics Non-Newtonian fluids